1. Field of the Invention
This invention relates generally to a speed limiting system for a windmill and more particularly to a speed limiting system utilizing a hysteresis effect to prevent the return of the system to its normal status until wind conditions have abated.
2. Description of the Prior Art
The use of windmills as a source of energy has been known for a long time. In recent years, due to the energy crisis there has been much renewed interest in this free energy source. However, several problems exist which prevent the windmill from becoming an effective energy source. One problem long recognized in the art is the effect of high winds, such as is found in gales and storms, on the blades and other equipment of the windmill. As the winds increase in velocity, the blades turn at a high rate of speed to the point that damage may be done to the equipment. Similarly, when electrical generators or other equipment driven by the windmill require a constant speed to operate at maximum capacity, even lesser winds may cause the equipment to operate at less than maximum capacity. Hence, it is necessary to limit the speed of the blades even though the point of damage has not been reached. Over the years, various adjusting mechanisms have been developed to counteract this effect. However, these various devices generally have not been satisfactory, due to their complex nature, their unreliability and their high degree of maintenance.
One known method of combating the problem has been to change the angle of the blades so that the high velocity winds are not able to change most of their energy into the rotational energy of the windmill. One early example of this type of device is shown in U.S. Pat. No. 26,345. The blades in this device are connected together in four groups. Each group contains an arm having a slide attached thereto. As the windmill turns faster due to the increasing velocity of the wind, the slide is hurled outwardly along the arm due to centrifugal force. However, each slide is connected to a central portion of the wheel by means of a rod. The rods are connected to a central cylinder which is springloaded. As the slides move outwardly, the rods pull the cylinder in a circular fashion and cause the blades of the windmill to present a larger surface to the incoming wind. Hence, the force of the wind no longer drives the windmill efficiently and the wheel slows down, thus avoiding damage.
Many similar solutions to this problem have been presented, which act to turn the blades of the windmill or in some other manner brake the blades. U.S. Pat. No. 2,058,500 shows such an arrangement where a part of the blade is rotated in relation to the main part of the blade so as to be almost perpendicular to the direction of travel of the blade and hence create more resistance to the movement of air. This effectively slows down the blade and avoids damage. The sideways movement of the blade tip is created when centrifugal force acts on the tip and forces it outwardly. Since the tip is mounted on a pin which has limited movement within an inclined slot, the outward movement of the blade tip also creates a sideways movement bringing the tip to a direction perpendicular to that of the blade.
A similar device is shown in U.S. Pat. No. 2,037,528, wherein the blade tips are again turned to act as a brake. In this device, the centrifugal force of the blade tip forces a spiral strap through a nut and thus causes a twisting motion as the blade tip moves outwardly under centrifugal force. The sideways movement of the blade acts as a brake for the windmill.
While these and other similar devices are somewhat effective in alleviating the effects of strong winds, there are still certain disadvantages which exist. When conventional devices are used, the blade or blade tips return to their original position as soon as the windmill decreases in speed. Thus, during a storm, when it would be desirable to have the windmill move at minimum speed, it nevertheless only slows down slightly before the brake is released and the windmill is allowed to speed up again. This cyclical fast and slow motion somewhat averages out but still allows the windmill to move faster than desired. An additional problem is that if the braking action is not applied simultaneously to all of the blades, there is an uneven force being applied to the windmill. This may result in damage to the faster moving blade or may create vibrations through the mill mechanism which may damage other parts. Also, there is no provision for allowing minor wind gusts to turn the blades without braking.